Optical recording medium substrate, optical recording medium, and method of manufacturing optical recording medium substrate

ABSTRACT

There is provided an optical recording medium substrate including a polycarbonate in which an average molecular weight is in a range from 15000 to 16000, and mass per unit volume at 25° C. is in a range from 1.1930 g/cm 3  to 1.2000 g/cm 3 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Priority PatentApplication JP 2012-251413 filed Nov. 15, 2012, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

The present disclosure relates to an optical recording medium substratethat reduces warpage, an optical recording medium using the opticalrecording medium substrate, and a method of manufacturing the opticalrecording medium substrate.

An optical recording medium is very excellent as a medium for supplyingsoftware (including music, videos, etc.), and has become widespread in acomprehensive range of fields due to the convenience of handling, theease of mass production, the reduction of manufacturing cost, etc. Inaddition, improvement in high density of an optical recording medium isin progress, and a Blu-ray disc (registered trademark) or the likeimplementing a significantly high capacity compared to, for example, acompact disc (CD) and a digital versatile disc (DVD) of the related art,has become widespread.

When these optical recording media are warped, it is difficult toexhibit good reproduction characteristics. This is because aberrationoccurs in a light spot.

In Japanese Unexamined Patent Application Publication No. 2009-271970,technology for laminating an optical transmission layer (transparentcover layer) formed by an active energy line curable resin havingpredetermined characteristics in order to suppress the warpage of anoptical recording medium (particularly, a Blu-ray disc) has beendisclosed.

SUMMARY

Incidentally, when the optical recording medium of the related art doesnot have sufficient rigidity and, for example, is stored for a longperiod of time in a horizontal state in which both ends of the opticalrecording medium between which the center is interposed are insertedinto holding grooves of a storage cartridge, there is a possibility ofwarpage occurring and it is difficult to exhibit good reproductioncharacteristics. In storage in a high-temperature environment, this maybe remarkably shown. This is one factor shortening a life of the opticalrecording medium. If the warpage of the optical recording medium storedin such a horizontal state can be reduced, this will greatly contributeto long-term storage of the optical recording medium.

It is desirable to provide a long-life optical recording mediumsubstrate which reduces warpage and has good reproductioncharacteristics even when an optical recording medium is stored for along period of time in a normal- or high-temperature environment in ahorizontal state, and an optical recording medium using the same.

According to an embodiment of the present disclosure, there is providedan optical recording medium substrate including a polycarbonate in whichan average molecular weight is in a range from 15000 to 16000, and massper unit volume at 2520 C. is in a range from 1.1930 g/cm³ to 1.2000g/cm³.

According to another embodiment of the present disclosure, there isprovided an optical recording medium including an optical recordingmedium substrate which is a polycarbonate in which an average molecularweight is in a range from 15000 to 16000, and mass per unit volume at25° C. is in a range from 1.1930 g/cm³ to 1.2000 g/cm³.

Further, according to still another embodiment of the presentdisclosure, there is provided a method of manufacturing an opticalrecording medium substrate, including filling and injection-molding apolycarbonate in which an average molecular weight is in a range from15000 to 16000 in a mold, and performing cooling in the mold for 6seconds or more.

Thereby, high rigidity of the optical recording medium is secured.According to one or more embodiments of the present disclosure, evenwhen an optical recording medium is stored for a long period of time,warpage due to its own weight can be suppressed and music, videos, etc.recorded on the optical recording medium can be reproduced in a goodstate.

In addition, even when the optical recording medium is stored in ahigh-temperature environment, it is possible to suppress warpage due toits own weight and similarly obtain good reproduction characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of a layer structure of an opticalrecording medium of an embodiment;

FIG. 2 is a diagram illustrating a method of measuring a density of anoptical recording medium substrate;

FIG. 3 is a diagram illustrating a change in a shape when the opticalrecording medium is held in a horizontal state;

FIG. 4 is a diagram illustrating a measurement result of warpage of theoptical recording medium when stored in the horizontal state;

FIG. 5 is a diagram illustrating a relationship between a density andwarpage of the optical recording medium substrate; and

FIG. 6 is a diagram illustrating a relationship between a cooling timein injection molding of the optical recording medium substrate and adensity.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

Hereinafter, contents of the present disclosure will be described in thefollowing order.

1. Structure of Optical recording medium

2. Manufacturing method

3. Density measuring method

4. Experiment results

5. Conclusion

Hereinafter, an optical recording medium, an optical recording mediumsubstrate, and a method of manufacturing the optical recording mediumsubstrate according to an embodiment will be described with reference toFIG. 1.

Technology of the present disclosure is applied to the optical recordingmedium substrate described here and applied to an optical recordingmedium using the optical recording medium substrate.

The present disclosure includes an example of a Blu-ray disc as theoptical recording medium, an example of a Blu-ray disc substrate as theoptical recording medium substrate, and an example of a method ofmanufacturing the Blu-ray disc substrate as the method of manufacturingthe optical recording medium substrate as will be described hereinafter.

A range of application of the present disclosure is not limited to anoptical recording medium serving as the Blu-ray disc, a Blu-ray discsubstrate, and a method of manufacturing the Blu-ray disc substrate,and, for example, can be widely applied to various optical recordingmedia such as a CD and a DVD, a substrate for various optical recordingmedia, and a method of manufacturing the substrate for various opticalrecording media.

1. Structure of Optical Recording Medium

In FIG. 1, an example of a layer structure of the Blu-ray disc (opticalrecording medium 1) is included.

FIG. 1A illustrates a layer structure of an optical recording medium 1in which a so-called recording layer 5 is a single layer.

The optical recording medium 1 has an optical recording medium substrate4 having one side on which the recording layer 5 is formed. As theoptical recording medium substrate 4, a polycarbonate is used. In thepolycarbonate, an average molecular weight is in a range from 15000 to16000 and mass per unit volume at 25° C. is in a range from 1.1930 g/cm³to 1.2000 g/cm³.

The recording layer 5 is formed in a concave/convex shape as a pit arraypattern including pits and spaces. The optical recording mediumsubstrate 4, for example, is formed with a thickness of about 1.1 mm.

A reflective film 6 for reflecting laser light 9 is formed forreproduction on the side having the recording layer 5 of the opticalrecording medium substrate 4 having the pit array pattern. A sideopposite the recording layer 5 in the reflective film 6 serves as anincidence surface of the laser light 9 on which light is collected by anobjective lens 14.

Although silver, a silver alloy, aluminum, or an aluminum alloy isusually used as the reflective film 6, the present disclosure is notlimited thereto because the function is satisfied as long as light of405 nm which is a wavelength of reproduction laser light can beefficiently reflected.

A transparent cover layer 7 having uniform and excellent surfacesmoothness is formed on an incidence surface side of the laser light 9in the reflective film 6, that is, an information readout surface side.Further, a hard coat layer 8 serving as a protective layer is formed ona surface side of the transparent cover layer 7, that is, a side onwhich the laser light 9 is incident.

In this case, although the protective layer is formed by the transparentcover layer 7 and the hard coat layer 8, a total thickness of thetransparent cover layer 7 and the hard coat layer 8, for example,becomes 95 to 105 μm. Although a ratio between the transparent coverlayer 7 and the hard coat layer 8 is not particularly regulated, forexample, the thickness of the hard coat layer 8 is usually 1.5 to 5 μm.

In addition, when viewed from an incidence side of the laser light 9 ofthe optical recording medium substrate 4, a surface (so-called labelsurface) of an opposite side is formed as a printing side 3 on which alabel printing operation for displaying contents of the opticalrecording medium 1 has been performed.

The hard coat layer 8 may not necessarily be formed. For example, when asurface protection function by the transparent cover layer 7 isobtained, an example in which no hard coat layer 8 is formed may beconsidered. A disc manufacturing process can be simplified by omittingthe formation of the hard coat layer 8. In the Blu-ray disc, a two-layerdisc structure having two recording layers 10 and 12 is alsostandardized. In FIG. 1B, a layer structure of a two-layer opticalrecording medium 2 is illustrated. As illustrated in FIG. 1B, theoptical recording medium substrate 4 is provided as in the opticalrecording medium 1. As described above, a polycarbonate is used as theoptical recording medium substrate 4. In the polycarbonate, an averagemolecular weight is in a range from 15000 to 16000 and mass per unitvolume at 25° C. is in a range from 1.1930 g/cm³ to 1.2000 g/cm³.

On an incidence surface side of the laser light 9 of the opticalrecording medium substrate 4, a first recording layer 10 and a secondrecording layer 12 are formed via an intermediate layer 11.

The first recording layer 10 and the second recording layer 12 areformed in a concave/convex shape as a pit array pattern including pitsand spaces. The intermediate layer 11, for example, is formed with athickness of 20 to 30 μm.

The reflective film 6 is formed between the first recording layer 10 andthe intermediate layer 11. A semi-transparent reflective film 13 isformed on a surface of the second recording layer 12. On the surface,the transparent cover layer 7 is formed. Further, the hard coat layer 8serving as the protective layer is formed on a surface side of thetransparent cover layer 7, that is, on a side on which the laser light 9is incident. A total thickness of the transparent cover layer 7 and thehard coat layer 8, for example, is 75 μm.

As in the case of the optical recording medium 1, the hard coat layer 8may not necessarily be formed.

The second recording layer 12 is formed by pressing a stamper on which apit array pattern is pre-formed against the intermediate layer 11.

Because the Blu-ray disc (optical recording media 1 and 2) has aone-side reading method, that is, the transparent cover layer 7 only onone side of the optical recording medium substrate 4, as illustrated inFIG. 1, the Blu-ray disc is asymmetric in a thickness direction and iseasily deformed due to the effect of a residual stress of each layer.

Therefore, the Blu-ray disc is constituted of a polycarbonate in whichan average molecular weight is in a range from 15000 to 16000 and massper unit volume at 25° C. is in a range from 1.1930 g/cm³ to 1.2000g/cm³, so that the occurrence of warpage is particularly efficientlysuppressed and good reproduction characteristics can be obtained.

2. Manufacturing Method

Hereinafter, a method of manufacturing the optical recording medium 1will be briefly described.

(a) First, injection molding is performed to create the opticalrecording medium substrate 4. The injection molding is performed bymelting polycarbonate pellets at a high temperature and filling themelted polycarbonate pellets in the mold.

(b) Next, the optical recording medium substrate 4 is cooled for a fixedtime within a mold and extracted from the mold. Thereby, the opticalrecording medium substrate 4 with a thickness of about 1.1 mm is formed.

(c) Next, the recording layer 5 and the reflective film 6 are laminatedon one side of the optical recording medium substrate 4 extracted fromthe mold using techniques such as sputtering, vapor deposition, andcoating.

(d) Subsequently, the transparent cover layer 7 is formed by aspin-coating method or laminating a film of about 100 μm on thereflective film 6.

(e) Next, the transparent cover layer 7 is hardened by radiatingultraviolet light to the transparent cover layer 7 formed by spincoating or film lamination.

(f) Ultimately, the hard coat layer 8 is formed.

A manufacturing method associated with injection molding for the mold isalso the same in manufacturing of a CD and a DVD.

Next, a method of manufacturing the two-layer optical recording medium 2will be briefly described.

The method of manufacturing the two-layer optical recording medium 2 issubstantially the same as in the case of the above-describedsingle-layer optical recording medium 1 until the first recording layer10 is formed.

After the first recording layer 10 is formed, the intermediate layer 11is formed. The intermediate layer 11 is hardened by radiatingultraviolet light thereto. On the intermediate layer 11, the secondrecording layer 12 and the semi-transparent reflective film 13 areformed by a sputtering method, etc. Thereafter, the transparent coverlayer 7 and the hard coat layer 8 are formed as in a single-layer disc.

In the technology of the present disclosure, injection molding isperformed by filling a polycarbonate of a range in which an averagemolecular weight is 15000 to 16000 in the mold. A cooling time withinthe mold is set to 6 seconds or more. By setting the cooling time to 6seconds or more, it is possible to have a characteristic that a densityof the polycarbonate is 1.1930 [g/cm³] to 1.2000 [g/cm³]. Ultimately,the optical recording medium substrate 4 is formed by a polycarbonate inwhich an average molecular weight is in a range from 15000 to 16000 anda density at 25° C. is in a range from 1.1930 [g/cm³] to 1.2000 [g/cm³].

Thereby, it is possible to suppress the deformation of the opticalrecording media 1 and 2, prevent warpage due to long-term storage, andobtain good reproduction characteristics as a result.

3. Density Measuring Method

Hereinafter, an experiment in a warpage state when the optical recordingmedium 1 was stored for a fixed period of time in an upper limit of ause temperature environment will be described. First, a method ofmeasuring the density of the optical recording medium substrate 4 usedin this experiment will be described with reference to FIG. 2.

As the density measuring method, an underwater substitution method wasused to accurately obtain the density even in a complex shape of theoptical recording medium substrate 4.

As a gravimeter for measuring the density, an electronic force balance30 capable of measuring up to a precision of 0.1 [mg] was used.

A classical balance or simply a balance is a measuring device whichobtains the mass of a sample from the mass of a balance weight when itis balanced by applying leverage, placing the sample on one side of aforce point or an action position, and placing the balance weightserving as a reference on the other side. On the other hand, in anelectronic force balance 30, a numerical value of weight is immediatelyobtained by merely placing a physical object to be measured without theneed of miscellaneous operations.

In addition, in the optical recording medium substrate 4 to be measured,an inorganic film through which water of 10 [nm] did not pass was formedon both sides of the optical recording medium substrate 4 in order tosuppress the degradation of precision of a density due to absorption.

First, the weight of the optical recording medium substrate 4 wasmeasured. The weight was measured by placing the optical recordingmedium substrate 4 on the electronic force balance 30 as illustrated inFIG. 2A. At this time, the weight was assumed to be m [g].

Next, the weight of a container 31 containing water 32 (pure water) wasmeasured as illustrated in FIG. 2B. At this time, the weight was assumedto be M1 [g].

Further, the weight was measured when the optical recording mediumsubstrate 4 was hung and floated by a support rod 33 under the water 32as illustrated in FIG. 2C. At this time, the weight was assumed to be M2[g].

From the above-described measurement results, when a density of watercorresponding to a water temperature at the time of weight measurementwas ρ [g/cm³], (M2−M1)/ρ was the volume of the optical recording mediumsubstrate 4. A density A of the optical recording medium substrate 4could be obtained by dividing m, which is the weight of the opticalrecording medium substrate 4, by (M2−M1)/ρ.

Ultimately, an expression for obtaining the density A [g/cm³] of theoptical recording medium substrate 4 is as follows.

A=ρm/(M2−M1).

According to the above method, the density of the optical recordingmedium substrate 4 was obtained.

4. Experiment Results

Next, the experiment results of a warpage state when a general opticalrecording medium was stored for a fixed period of time in an upper limitof a usage environment temperature and a warpage state of the opticalrecording medium 1 having the optical recording medium substrate 4 ofthis embodiment will be described with reference to FIGS. 3 to 6.

FIG. 3 visually illustrates a storage state and a deformation state ofthe general optical recording medium (optical disc) and illustrates adeformation state when the optical recording medium was supported at twopoints and stored in the horizontal state. FIG. 3A corresponds to thecase in which there is no deformation and no warpage occurs.

FIG. 3B illustrates a deformation state when storage was performedwithin a constant-temperature layer at a temperature of 55° C. for 96hours. For ease of understanding, FIG. 3B is illustrated in anexaggerated manner. Although portions (arrows) supported as illustratedin FIG. 3B are not deformed, the other portions are bent and drooped dueto their own weight or deformed in an overall potato chip shape. In thiscase, a recording or reproduction operation using a laser is not normal.This deformation is a cause of malfunction.

FIG. 4 is a graph obtained by specifically measuring a deformationamount (warpage) of the general optical recording medium illustrated inFIG. 3 as a tilt (angle) from the center of the optical recordingmedium. The horizontal axis represents a radius of the optical recordingmedium and the vertical axis represents the angle from the center of theoptical recording medium. This tilt (angle) corresponds to the warpageof the optical recording medium. In FIG. 4, □ represents a minimumvalue, ▴ represents an average value, and ♦ represent a maximum value.When the tilt exceeds ±0.3 degrees, recording or reproduction using alaser is impeded. This is because aberration occurs in a light spot(focus).

FIG. 4A corresponds to the case of FIG. 3A, and illustrates the tilt ofthe optical recording medium before storage (immediately aftermanufacturing). In this case, a difference between a maximum value and aminimum value of the tilt is in a range of ±0.2 degrees, which issubstantially flat. This is obviously within ±0.3 degrees. Accordingly,neither recording nor reproduction using laser is impeded.

FIG. 4B corresponds to the case of FIG. 3B, and is a graph illustratinga tilt of the optical recording medium after storage at a temperature of55° C. for 96 hours in a two-point support state. A difference betweenthe maximum value and the minimum value reaches ±0.6 degrees (a radiusof about 58 mm) as illustrated in FIG. 4B. In this case, a recording orreproduction operation is impeded, resulting in inconvenience.

FIG. 5 illustrates a relationship between a density and warpage (tilt)of the optical recording medium substrate 4 having a polycarbonate inwhich an average molecular weight is 15000 or 16000 as the opticalrecording medium substrate 4. Here, the density refers to mass per unitvolume.

While the warpage of the optical recording medium substrate 4 isabruptly increased in a region in which a density at a temperature of25° C. is 1.1926 [g/cm³] or less as illustrated in FIG. 5, a warpageamount is gently decreased in a region in which the density is above1.1926 [g/cm³]. When the density is 1.1920 [g/cm³], the warpage amountis about 0.56 degrees.

On the other hand, when the density at the temperature of 25° C. exceeds1.1926 [g/cm³], the warpage amount is decreased to 0.28 degrees or less.Therefore, the use of a polycarbonate in which the density is 1.1930[g/cm³] or more for the optical recording medium substrate 4 maysufficiently suppress the warpage and may be practically preferred.

From the above-described measurement results, if a polycarbonate inwhich an average molecular weight is in a range from 15000 to 16000 andmass per unit volume at 25° C. is 1.1930 [g/cm³] or more is used as theoptical recording medium substrate 4, the warpage can be reduced andgood recording or reproduction characteristics of the optical recordingmedia 1 and 2 can be obtained.

The density of the polycarbonate can be controlled under appropriateconditions such as molding conditions and an annealing process. Here,the density can be set to a predetermined value by setting a coolingtime within the mold to, for example, 6 seconds, as will be describedlater.

In addition, an upper limit of the density of the polycarbonate isgenerally 1.2000 [g/cm³]. Therefore, the polycarbonate of the presentdisclosure in which an average molecular weight is in a range from 15000to 16000 and mass per unit volume at 25° C. is in a range from 1.1930[g/cm³] to 1.2000 [g/cm³] is preferred.

FIG. 6 is a diagram illustrating a relationship between a cooling timewithin a mold of the optical recording medium substrate 4 and a density.The horizontal axis represents the cooling time and the vertical axisrepresents the density. When the cooling time is increased asillustrated in FIG. 6, the density of the optical recording mediumsubstrate 4 is increased. If the cooling time is 6 seconds or more, thedensity of the optical recording medium substrate 4 can be set to 1.1930g/cm³ or more.

Therefore, it is possible to suppress the occurrence of warpage of theoptical recording medium substrate 4 and the optical recording medium 1having the same by setting the cooling time within the mold to 6 secondsor more upon molding the optical recording medium substrate 4.

5. Conclusion

According to the above, when an optical recording medium substrate whichis a polycarbonate in which an average molecular weight is in a rangefrom 15000 to 16000and mass per unit volume at 25° C. is in a range from1.1930 g/cm³ to 1.2000 g/cm³ is used for the optical recording medium,the occurrence of warpage is particularly efficiently suppressed andgood reproduction characteristics can be obtained.

In addition, it is possible to prevent the deformation of a disc in a CDand DVD as well as a Blu-ray disc, prevent the warpage of the disc dueto long-term storage, and obtain good reproduction characteristics. Itshould be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An optical recording medium substrate comprising:a polycarbonate in which an average molecular weight is in a range from15000 to 16000, and mass per unit volume at 25° C. is in a range from1.1930 g/cm³ to 1.2000 g/cm³.
 2. An optical recording medium comprising:an optical recording medium substrate which is a polycarbonate in whichan average molecular weight is in a range from 15000 to 16000, and massper unit volume at 25° C. is in a range from 1.1930 g/cm³ to 1.2000g/cm³.
 3. The optical recording medium according to claim 2, furthercomprising: a transparent cover layer on an information readout surfaceside of the optical recording medium substrate.
 4. A method ofmanufacturing an optical recording medium substrate, comprising: fillingand injection-molding a polycarbonate in which an average molecularweight is in a range from 15000 to 16000 in a mold; and performingcooling in the mold for 6 seconds or more.
 5. The method ofmanufacturing an optical recording medium substrate according to claim4, comprising: forming a transparent cover layer on an informationreadout surface side of the optical recording medium substrate afterextracting the optical recording medium substrate from the mold.